Effects of electroacupuncture on the neuronal activity of the arcuate nucleus of the rat hypothalamus

Electroacupuncture (EA) effects on the spontaneous unit activities of 143 neurons in the hypothalamic arcuate nucleus (ARH) which were electrophysiologically identified to project to the median eminence were investigated using anesthetized rats weighing about 300 g. Stimuli were delivered unilaterally to a meridian point of Ho-Ku in the forepaw as rectangular pulses of 5 ms duration, intensity 300 to 500 microA, for 15 min at 3 and 45 Hz. Unit activities of ARH cells were extracellularly recorded and the mean firing rates were compared before and after EA stimulation. Stimulation at 3 or 45 Hz induced a long-lasting and naloxone-reversible suppression of the magnitude of the digastric electromyogram (dEMG) in the jaw opening reflex to 48 to 56% of the control value. Based upon the EA effects on the spontaneous firing rate, the ARH cells were classified into three types: the rate either increased (type I), decreased (type II), or did not change (type III). Type I, II, and III neurons composed 56, 40, and 4% of the recorded neurons (N = 45) when EA stimulation was applied at 3 Hz, and 27, 70, and 3% (N = 37) at 45 Hz, respectively. The distribution of the three types of ARH neurons after EA stimulation at 3 Hz was significantly different (P less than 0.05, chi-square test) from that at 45 Hz.     

Effects of lesion and stimulation of rat hypothalamic arcuate nucleus on the pain system

Electrolytic lesion and stimulation of the arcuate nucleus of the hypothalamus (ARH) were investigated in Wistar albino rats. 1) Discrete lesion of the ARH is followed by hyperalgesia as manifested by significantly high values of pain rating in the formalin test and by depressed analgesic effect of electroacupuncture stimulation on digastric electromyogram (dEMG) activity. 2) ARH stimulation appreciably suppressed both the responses of neurons in the lateral hypothalamus (LHA) and dEMG activity induced by tooth pulp stimulation. The suppressive influence of ARH stimulation on the responses of LHA neurons was antagonized by intraperitoneally injected and electrophoretically applied naloxone. It is thus suggested that the ARH is involved in induction or modulation of opioid-mediated analgesia.     

Analgesic electrical stimulation of the hypothalamic arcuate nucleus: tolerance and its cross-tolerance to 2 Hz or 100 Hz electroacupuncture

Focal electrical stimulation of the arcuate nucleus of the hypothalamus (ARH) for 5 min (1 session) produced a marked elevation of tail flick latency (TFL) to noxious heat in the pentobarbital-anesthetized rat. Repeated stimulation for a total of 11 sessions at 30 min intervals resulted in a gradual decline in the hypoalgesic action, and this tolerance may last for 7 days. Tolerance to the ARH analgesic stimulation reduced the analgesia produced by low (2 Hz) but not high (100 Hz) frequency electroacupuncture (EA); and tolerance to low frequency EA analgesia attenuated the ARH stimulation-produced analgesia without affecting high frequency EA analgesia. Alternatively, rats tolerant to high-frequency EA analgesia were still sensitive to either the ARH or low-frequency EA stimulation. These results suggest that the ARH stimulation and low-frequency EA administration produced analgesia via a common neural mechanism, supporting our hypothesis put forward previously that the ARH plays an important role in mediating low- but not high-frequency EA analgesia.     

Long-term high-frequency electro-acupuncture stimulation prevents neuronal degeneration and up-regulates BDNF mRNA in the substantia nigra and ventral tegmental area following medial forebrain bundle axotomy

Electroacupuncture (EA) has been used in China for many years to treat Parkinson's disease (PD) with reportedly effective results. However, the physiological and biological mechanism behind its effectiveness is still unknown. In the present study, different frequencies of chronic EA stimulation (0, 2, 100 Hz) were tested in a partially lesioned rat model of PD which was induced by transection of the medial forebrain bundle (MFB). After 24 sessions of EA stimulation (28 days after MFB transection), dopaminergic neurons in the ventral midbrain were examined by immunohistochemical staining, and brain-derived neurotrophic factor (BDNF) mRNA levels in ventral midbrain were measured by in situ hybridization. The results show a marked decrease of dopaminergic neurons on the lesioned side of the substantia nigra (SN) comparing with the unlesioned side. Zero Hz and 2 Hz EA stimulation had no effect on the disappearance of dopaminergic neurons. However, after 100 Hz EA, about 60% of the tyrosine hydroxylase (TH)-positive neurons remained on the lesioned side of the SN. In addition, levels of BDNF mRNA in the SN and ventral tegmental area (VTA) of the lesioned side were significantly increased in the 100 Hz EA group, but unchanged in the 0 and 2 Hz groups. Our results suggest that long-term high-frequency EA is effective in halting the degeneration of dopaminergic neurons in the SN and up-regulating the levels of BDNF mRNA in the subfields of the ventral midbrain. Activation of endogenous neurotrophins by EA may be involved in the regeneration of the injured dopaminergic neurons, which may underlie the effectiveness of EA in the treatment of PD.     

The arcuate nucleus of hypothalamus mediates low but not high frequency electroacupuncture analgesia in rats

Electrolytic, kainic acid or sham lesions were made in the arcuate nucleus of the hypothalamus (ARH) in female Wistar rats to investigate the putative role of the ARH in the organization of low (2 Hz) or high (100 Hz) frequency electroacupuncture (EA) analgesia. Both electrolytic and chemical lesions lead to an almost total suppression of the low frequency EA analgesia as measured 4 and 6 days following the surgical intervention, leaving high frequency EA analgesia unaffected. In sham-operated animals, the antinociceptive effect induced by low or high frequency EA was essentially intact. These data indicate that neurones of the ARH most likely play an important role in mediating low, but not high frequency EA analgesia.     

Electroacupuncture modifies the expression of c-fos in the spinal cord induced by noxious stimulation.

The present study was designed to investigate the effect of 4 Hz vs. 100 Hz electroacupuncture (EA) on c-fos expression in the spinal cord induced by noxious stimulation (NS). A second objective was to evaluate the sensitivity of these two different frequencies of EA stimulation to the opiate antagonist, naloxone. Mechanical NS was applied to the right hindpaw following 30 min of either 4 Hz or 100 Hz EA treatment and the resulting c-fos expression in the spinal cord dorsal horn was compared to that obtained in rats exposed only to the noxious stimulation. The involvement of endogenous opioids in the EA response to 4 Hz or 100 Hz stimulation frequencies was evaluated by pretreating rats with naloxone (5 mg/kg, i.p.) 10 min prior to EA. Both 4 Hz and 100 Hz EA reduced the number of c-fos-immunoreactive neurons in the spinal dorsal horn induced by noxious stimulation by 58% and 50%, respectively. The suppression of c-fos expression induced by 4 Hz EA was completely reversed by prior treatment with naloxone. On the other hand, the suppression of c-fos induced by 100 Hz EA was only partially blocked by this opiate antagonist. These data indicate that both high- and low-frequency EA are capable of inhibiting the expression of c-fos in the dorsal horn induced by NS. Low-frequency EA appears to be mediated primarily by endogenous opioid systems, while non-opioid mechanisms may be involved in mediating the analgesic effect of high frequency EA. These results support the hypothesis that EA has a direct inhibitory effect on spinal cord dorsal horn neurons and extend the results of previous studies which indicate low frequency EA is mediated by opiate sensitive circuitry, while high frequency EA is predominantly mediated by non-opioid neurotransmitters.     

Long-term synaptic plasticity in the spinal dorsal horn and its modulation by electroacupuncture in rats with neuropathic pain

Our previous study has reported that electroacupuncture (EA) at low frequency of 2 Hz had greater and more prolonged analgesic effects on mechanical allodynia and thermal hyperalgesia than that EA at high frequency of 100 Hz in rats with neuropathic pain. However, how EA at different frequencies produces distinct analgesic effects on neuropathic pain is unclear. Neuronal plastic changes in spinal cord might contribute to the development and maintenance of neuropathic pain. In the present study, we investigated changes of spinal synaptic plasticity in the development of neuropathic pain and its modulation by EA in rats with neuropathic pain. Field potentials of spinal dorsal horn neurons were recorded extracellularly in sham-operated rats and in rats with spinal nerve ligation (SNL). We found for the first time that the threshold for inducing long-term potentiation (LTP) of C-fiber-evoked potentials in dorsal horn was significantly lower in SNL rats than that in sham-operated rats. The threshold for evoking the C-fiber-evoked field potentials was also significantly lower, and the amplitude of the field potentials was higher in SNL rats as compared with those in the control rats. EA at low frequency of 2 Hz applied on acupoints ST 36 and SP 6, which was effective in treatment of neuropathic pain, induced long-term depression (LTD) of the C-fiber-evoked potentials in SNL rats. This effect could be blocked by N-methyl-d-aspartic acid (NMDA) receptor antagonist MK-801 and by opioid receptor antagonist naloxone. In contrast, EA at high frequency of 100 Hz, which was not effective in treatment of neuropathic pain, induced LTP in SNL rats but LTD in sham-operated rats. Unlike the 2 Hz EA-induced LTD in SNL rats, the 100 Hz EA-induced LTD in sham-operated rats was dependent on the endogenous GABAergic and serotonergic inhibitory system. Results from our present study suggest that (1) hyperexcitability in the spinal nociceptive synaptic transmission may occur after nerve injury, which may contribute to the development of neuropathic pain; (2) EA at low or high frequency has a different effect on modulating spinal synaptic plasticities in rats with neuropathic pain. The different modulation on spinal LTD or LTP by low- or high-frequency EA may be a potential mechanism of different analgesic effects of EA on neuropathic pain. LTD of synaptic strength in the spinal dorsal horn in SNL rats may contribute to the long-lasting analgesic effects of EA at 2 Hz.     

Effects of electroacupuncture on cold allodynia in a rat model of neuropathic pain: mediation by spinal adrenergic and serotonergic receptors

The present study was performed to examine the effects of electroacupuncture (EA) on cold allodynia and its mechanisms related to the spinal adrenergic and serotonergic systems in a rat model of neuropathic pain. For the neuropathic surgery, the right superior caudal trunk was resected at the level between S1 and S2 spinal nerves innervating the tail. Two weeks after the nerve injury, EA stimulation (2 or 100 Hz) was delivered to Zusanli (ST36) for 30 min. The behavioral signs of cold allodynia were evaluated by the tail immersion test [i.e., immersing the tail in cold water (4 degrees C) and measuring the latency to an abrupt tail movement] before and after the stimulation. And then, we examined the effects of intrathecal injection of prazosin (alpha1-adrenoceptor antagonist, 30 microg), yohimbine (alpha2-adrenoceptor antagonist, 30 microg), NAN-190 (5-HT1A antagonist, 15 microg), ketanserin (5-HT2A antagonist, 30 microg), and MDL-72222 (5-HT3 antagonist, 12 microg) on the action of EA stimulation. Although both 2 Hz and 100 Hz EA significantly relieved the cold allodynia signs, 2 Hz EA induced more robust effects than 100 Hz EA. In addition, intrathecal injection of yohimbine, NAN-190, and MDL-72222, but not prazosin and ketanserin, significantly blocked the relieving effects of 2 Hz EA on cold allodynia. These results suggest that low-frequency (2 Hz) EA is more suitable for the treatment of cold allodynia than high-frequency (100 Hz) EA, and spinal alpha2-adrenergic, 5-HT1A and 5-HT3, but not alpha1-adrenergic and 5-HT2A, receptors play important roles in mediating the relieving effects of 2 Hz EA on cold allodynia in neuropathic rats.     

Respiratory responses elicited by rostral versus caudal dorsal periaqueductal gray stimulation in rats

The periaqueductal gray (PAG) is a central neural region essential for defense behavior and coordination of accompanying autonomic responses. Activation of rostral versus caudal dorsal (dPAG) regions mediates different cardiovascular response patterns. Stimulation of the dPAG also elicits increased respiratory activity, however, it is unknown if there is a regional difference in dPAG modulation of respiratory pattern. The present study was undertaken to identify whether activation of rostral vs caudal dPAG modulates respiration differently. In anesthetized, spontaneously breathing rats, chemical and electrical stimulation in rostral and caudal dPAG evoked an increased respiratory frequency (f(R)) with significant shortening of both inspiratory (Ti) and expiratory time (Te). Stimulation in the dPAG also evoked significant increases in electromyography activity of the diaphragm (dEMG), arterial pressure, and heart rate. Caudal dPAG stimulation evoked a greater increase in f(R) due to a significantly greater decrease in Ti and Te than the rostral dPAG. Caudal dPAG stimulation also evoked a greater increase in baseline dEMG activity and elicited a significantly greater increase in dEMG amplitude above baseline than rostral dPAG. There was a rostro-caudal difference in the post-stimulus respiratory recovery response, with the caudal dPAG eliciting a longer sustained effect. No regional differences were identified in the arterial blood pressure and heart rate during dPAG stimulation. The results demonstrate that the magnitude of the respiratory response during and immediately after activation of the caudal dPAG is greater than during rostral dPAG stimulation.     

Volume expansion suppresses the tooth-pulp evoked jaw-opening reflex related activity of trigeminal neurons in rats

The aim of the present study is to clarify whether physiological stimulation of vagal afferents modulates the activity of the trigeminal spinal nucleus oralis (TSNO) neurons related to the tooth-pulp (TP)-evoked jaw-opening reflex (JOR) in pentobarbital-anesthetized rats. The activity of TSNO neurons and the amplitude of digastric electromyogram (dEMG) increased proportionally during 1.0-3.5 times the threshold for JOR. The amplitude of the dEMG of 14 out of 17 rats was suppressed by physiological stimulation of vagal afferents after intravenous infusion of Ficoll. Out of 23, 18 TSNO unit activities in 14 rats were also suppressed by Ficoll infusion. This suppressive effect of unit and dEMG activities returned to the control level within 25 min. After administration of naloxone (0.5 and 1.0 mg/kg, i.v.) the suppressive effect of Ficoll infusion on the activity of TSNO neurons (5/7) was significantly attenuated compared to the control (p < 0.01). The inhibition TSNO neuronal and dEMG activities by Ficoll infusion was volume-dependent in a range of 5-10% of total blood volume. Furthermore, right vagus nerve ligation greatly inhibited the suppressive effect of Ficoll-induced TSNO activity. These results therefore suggest that low-pressure cardiopulmonary baroreceptors whose afferents travel in the vagus nerve inhibit the pulpal nociceptive transmission.     

Differential modulation of anterior cingulate cortical activity by afferents from ventral tegmental area and mediodorsal thalamus

A distinct increase in cell firing activity is reported in prefrontal cortex during working memory tasks. The afferents that modulate this activity are not yet identified. Using in vivo intracellular recording and labelling of prefrontal cortical pyramidal neurons in anaesthetized rats, we systematically evaluated the influences of afferent projections arising from the ventral tegmental area (VTA) and mediodorsal thalamus (MD) by phasic electrical stimulation with a range of stimulus frequencies. Both VTA- and MD-responsive pyramidal neurons exhibited extensive intracortical axon arborization. Neither single shocks to the VTA at 0.2 Hz, nor low frequency trains of stimuli at 1-4 Hz (< 5 Hz) interrupted the periodicity of membrane bistability in bistable pyramidal neurons. However, high-frequency VTA-train stimulation (10-50 Hz) interrupted the bistability, and produced sustained membrane depolarizations accompanied by intense tonic firing in a frequency-dependent manner. Electrical stimulation of MD (10-50 Hz) did not produce sustained activity in the same PFC neurons. Thus, the sustained activity induced by high-frequency VTA trains is input selective. This effect of VTA-train stimulation was attenuated by systemic injection of the D1 receptor antagonist, SCH 23390, and blocked by acute dopamine (DA) depletion produced via alpha-methyl-para-tyrosine pre-treatment, suggesting that sustained cortical activity is mediated by DA. Chemical stimulation of VTA via intra-VTA infusion of NMDA induced sustained activity similar to VTA-train stimulation. Thus, while both VTA- and MD-responsive pyramidal neurons exhibited extensive intracortical axon arborization, VTA synapses (as opposed to MD synapses) may be critically positioned in the dendritic arborizations of anterior cingulate cortical pyramidal neurons, which may allow their modulation of sustained activity in prefrontal bistable neurons.     

Dissociative anesthesia and striatal neuronal electrophysiology.

Compared with results obtained in locally anesthetized, paralyzed rats, the dissociative anesthetic ketamine did not alter either the number of spontaneously active striatal neurons or the basal firing rate of striatal neurons; 90% of these cells exhibited the type I striatal neuron waveform. Chloral hydrate anesthesia suppressed both the occurrence and the firing rate of spontaneously active type I cells, but did not alter the activity of type II striatal neurons. Cortical stimulation preferentially activated type II cells in paralyzed rats and in chloral hydrate-anesthetized rats. Thus, under dissociative anesthesia it is possible to study spontaneously active type I striatal neurons. However, a method of activation such as cortical stimulation is necessary to study type II striatal neurons.     

Wind-up of tooth pulp-evoked responses and its suppression in rat trigeminal caudal neurons

Induction and suppression of wind-up were studied in 97 tooth pulp-driven neurons in the trigeminal subnucleus caudalis, using Wistar albino rats anesthetized with urethane and alpha-chloralose. Tooth pulp stimulation applied to an ipsilateral lower incisor evoked early discharges, indicating excitatory inputs from A-delta fibers and subsequent late discharges from C-fiber volleys in caudal neurons. Wind-up was efficiently evoked by stimulation delivered at 0.3-1 Hz, with current intensity sufficient to evoke late discharges. Conditioning stimulation of the arcuate nucleus of the hypothalamus (ARH) suppressed late discharges, including wind-up, without affecting the A-fiber response. Focal cooling of the periaqueductal gray (PAG) abolished the suppression by the ARH and further enhanced the wind-up of the caudal neurons. These results suggest: 1) Temporal summation of depolarization evoked by C-fiber volleys builds wind-up in caudal neurons; 2) ARH stimulation suppresses late discharges by blocking synaptic transmission from C-fiber inputs, and this interrupts prolonged facilitation of the neurons; 3) the ARH is involved in induction of inhibitory controls descending from the PAG to the trigeminal caudalis.     

Suppressive effect of vagal afferents on the activity of the trigeminal spinal neurons related to the jaw-opening reflex in rats: involvement of the endogenous opioid system

The purpose of the present study is to test the hypothesis that via the endogenous pain control system, vagal afferent input modulates the activity of the trigeminal spinal nucleus oralis (TSNO) related to the tooth pulp (TP)-evoked jaw-opening reflex (JOR). Extracellular single-unit recordings were made from 36 TSNO units responding to TP electrical stimulation with a constant temporal relationship to a digastric electromyogram (dEMG) signal in 26 pentobarbital-anesthetized rats. The activity of 36 TSNO neurons and the amplitude of the dEMG increased proportionally during 1.0–3.5 times the threshold for JOR. Some of these neurons (4 out of 5) were also excited by chemical stimulation (bradykinin, 1–2 μl, 1 mM) of TP. In 31 out of 36 TSNO neurons (86%), their activities during tooth pulp stimulation were suppressed by conditioning stimulation of the right vagus nerve. The suppressive effect of vagal afferent stimulation occurred at conditioning-test intervals of 20–150 ms after the onset of the stimulation, and its maximal suppressive effect occurred at approximately 50 ms. The mean time course of this suppressive effect paralleled that of the dEMG. After administration of naloxone (0.5 and 1.0 mg/kg, i.v.), an opiate receptor blocker, the suppressive effect on the activity of TSNO neurons (6 out of 8) was significantly attenuated at the conditioning-test interval of 50 ms compared to the control (p < 0.01). These results suggested that vagal afferent input inhibits nociceptive transmission in the TSNO related to TP-evoked JOR and this inhibitory effect may occur via the endogenous opioid system in rats.     

Relieving effects of electroacupuncture on mechanical allodynia in neuropathic pain model of inferior caudal trunk injury in rat: mediation by spinal opioid receptors

The relieving effects of electroacupuncture (EA) on mechanical allodynia and its mechanism related to the spinal opioid system were investigated in a rat model of neuropathic pain. To produce neuropathic pain in the tail, the right superior caudal trunk was resected between the S1 and S2 spinal nerves. Two weeks after the surgery, EA stimulation (2 or 100 Hz, 0.3 ms, 0.2-0.3 mA) was delivered to Zusanli (ST36) for 30 min. The degree of mechanical allodynia was evaluated quantitatively by touching the tail with von Frey hair (2.0 g) at 10 min intervals. These rats were then subjected to an i.t. injection with one of the three specific opioid agonists in successive ways: the mu agonist (DAMGO 25, 50 and 100 pmol), the delta agonist (DADELT II 0.5, 1 and 2 nmol), and the kappa agonist (U50488H 5, 10 and 20 nmol) separated by 10 min in cumulative doses. During 30 min of EA stimulation, specific opioid antagonists were subjected to i.t. injection: the mu antagonist (beta-FNA 5, 10 and 20 nmol), the delta antagonist (naltrindole 5, 10 and 20 nmol), and the kappa antagonist (nor-BNI 3, 6 and 12 nmol) separated by 10 min in cumulative doses. As a result, EA reduced the behavioral signs of mechanical allodynia. Two Hz EA induced a robust and longer lasting effect than 100 Hz. All three opioid agonists also showed relieving effects on mechanical allodynia. However, nor-BNI could not block the EA effects on mechanical allodynia, whereas beta-FNA or naltrindole significantly blocked EA effects. These results suggest that the mu and delta, but not kappa, opioid receptors in the spinal cord of the rat, play important roles in mediating relieving effects on mechanical allodynia induced by 2 Hz EA.     

Prolonged inhibition of rostral ventral lateral medullary premotor sympathetic neurons by electroacupuncture in cats

We have shown that electroacupuncture (EA) at the Neiguan-Jianshi (N-J) acupoints over the median nerve reduces myocardial ischemia by modulating the pressor response induced by application of bradykinin on the gallbladder. The present study was designed to investigate the neural substrate underlying the prolonged modulatory effect of EA on visceral afferent input into the rostral ventral lateral medulla (rVLM). Experiments were performed on ventilated anesthetized cats. Neuronal activity was recorded while either stimulating the splanchnic nerve or applying EA at the N-J acupoints. Thirty-three cells responsive to splanchnic nerve and median nerve stimulation were antidromically driven from the intermediolateral columns, T(2)-T(4), indicating their function as premotor sympathetic neurons. These neurons also received baroreceptor input demonstrating that they were cardiovascular sympathoexcitatory cells. Arterial pulse-triggered averaging and coherence analysis demonstrated a correlation between cardiac-related discharge activity with 2.8+/-0.3 Hz rhythms and arterial blood pressure. Stimulation (2 Hz, 1-4 mA, 0.5 ms) of the splanchnic nerve for 30 s evoked excitatory responses. These neuronal responses were reduced during and after 30-min stimulation of EA at the Neiguan-Jianshi acupoints. These splanchnic nerve-induced excitatory responses in neurons subjected to 30 min of EA were reduced by 68%. Iontophoresis of naloxone promptly reversed the EA-induced inhibitory effect by 52%. Neuronal activity in the rVLM induced by splanchnic nerve stimulation was reduced for 50 (or more) min after termination of EA in 7 of 12 rVLM neurons.Our results indicate that rVLM premotor sympathetic cardiovascular neurons receive convergent input from the gallbladder through the splanchnic nerve and N-J acupoints through the median nerves. Through an opioid mechanism, EA inhibits splanchnic nerve-induced excitatory responses of these rVLM neurons. Many of these neurons receiving convergent visceral and somatic input exhibit long-lasting inhibition by EA.     

Involvement of opioid receptors in electroacupuncture-produced anti-hyperalgesia in rats with peripheral inflammation

Our previous study showed that electroacupuncture (EA) significantly attenuated inflammatory hyperalgesia. It has also been reported that EA analgesia in uninjured animals is mediated by mu and delta opioid receptors at 2-15 Hz and by kappa opioid receptor at 100 Hz. Because persistent pain changes neural response to external stimulation, we hypothesized that (1) the mechanisms of EA anti-hyperalgesia may be different under conditions of persistent pain and that (2) combining EA with a sub-effective dose of morphine could enhance EA anti-hyperalgesia. Hyperalgesia, decreased paw withdrawal latency (PWL) to a noxious thermal stimulus, was induced by subcutaneously injecting complete Freund's adjuvant (CFA) into the hind paws of rats. Selective antagonists against mu (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-ThrNH2, CTOP), delta (naltrinodole, NTI) and kappa (nor-binaltorphimine, BNI) opioid receptors were administered intrathecally 10 min before each of two EA treatments at acupoint Huantiao (GB30), one immediately post and the other 2 h post-CFA. Morphine was given (i.p.) 40 min before the second EA treatment. PWL was measured before and 2.5 and 5 h post-CFA. Both 10 and 100 Hz EA-produced anti-hyperalgesia were blocked spinally by mu- and delta- but not kappa-receptor antagonists. EA combined with a sub-threshold dose of morphine (2.5 mg/kg) enhanced anti-hyperalgesia additively (10 Hz EA) or synergistically (100 Hz EA) compared to that produced by each component alone. These results suggest selective involvement of mu and delta, but not kappa, receptors in EA-produced anti-hyperalgesia in rats. A combined EA and opioid drug protocol may provide an improved treatment strategy for inflammatory pain.     

Reciprocal modulation of somatosensory evoked N20m primary response and high-frequency oscillations by interference stimulation.

OBJECTIVES: We examined whether the inverse relation between somatic evoked N20m primary response and high-frequency oscillations during a wake-sleep cycle (Hashimoto, I., Mashiko, T., Imada, T., Somatic evoked high-frequency magnetic oscillations reflect activity of inhibitory interneurons in the human somatosensory cortex, Electroenceph clin Neurophysiol 1996;100:189-203) holds for interference stimulation. METHODS: Somatosensory evoked fields (SEFs) from 14 subjects were measured following electric median nerve stimulation at the wrist with, and without, concurrent brushing of the palm and fingers. SEFs were recorded with a wide bandpass (0.1-1200 Hz) and then N20m and high-frequency oscillations were separated by subsequent low-pass (< 300 Hz) and high-pass (> 300 Hz) filtering. RESULTS: The N20m decreased dramatically in amplitude during interference stimulation. In contrast, the high-frequency oscillations moderately increased in number of peaks. CONCLUSIONS: These results demonstrate the presence of an inverse relation between N20m and high-frequency oscillations for interference stimulation. We speculate that the high-frequency oscillations represent a localized activity of GABAergic inhibitory interneurons of layer 4, characterized by a high-frequency spike burst (200-1000 Hz) without adaptation, and that the continuous interference stimulation induces tonic excitation of the interneurons, leading to a facilitation of responses to the coherent afferent volley elicited by the median nerve stimulation (bottom-up mechanism). On the other hand, refractoriness of the pyramidal neurons caused directly by interference stimulation along with an enhanced feed-forward inhibition from the interneurons will lead to a decrease of N20m amplitude.     

Low-frequency electroacupuncture suppresses carrageenan-induced paw inflammation in mice via sympathetic post-ganglionic neurons, while high-frequency EA suppression is mediated by the sympathoadrenal medullary axis

Although the frequency-dependent antinociceptive mechanisms of electroacupuncture (EA) have been well demonstrated, the anti-inflammatory mechanisms that underlie the suppressive effects induced by different frequencies of EA stimulation on peripheral inflammation are largely unknown. We have previously reported that EA stimulation can activate the sympathetic nervous system (SNS) and that this activation is responsible for the EA-induced suppression of zymosan-induced leukocyte migration. The present study was designed to evaluate the differential effect of low (1Hz, LF EA) versus high (120Hz, HF EA) frequency EA stimulation on SNS activation and ultimately on carrageenan-induced inflammation. Immediately after carrageenan injection, we applied either LF EA or HF EA bilaterally to the Zusanli (ST36) acupoints. To evaluate the anti-inflammatory effect of EA (EA-AI), paw volume and myeloperoxidase (MPO) activity, a marker of infiltrated leukocytes, were measured and the paw withdrawal latency to noxious heat stimulation was also assessed. Both LF EA and HF EA significantly suppressed the carrageenan-induced paw edema and MPO activity. Moreover, thermal hyperalgesia was strongly attenuated in both the LF EA and HF EA groups. Adrenalectomy significantly diminished HF EA-AI without affecting LF EA-AI. Pretreatment with the corticosterone receptor antagonist, RU-486 did not affect either LF EA- or HF EA-AI. On the other hand, administration of 6-hydroxydopamine (a neurotoxin for peripheral sympathetic nerve endings) selectively blocked LF EA-AI. Propranolol (a beta-adrenoceptor antagonist) completely abolished both LF EA- and HF EA-AI. The results of this study suggest that the suppressive effects of LF EA on carrageenan-induced paw inflammation are mediated by sympathetic post-ganglionic neurons, while the suppressive effects of HF EA are mediated by the sympatho-adrenal medullary axis.